Throughout many regions of the world, heavy oil, a hydrocarbon material having much higher viscosity or lower API gravity (less than 20° API, typically 7° to 12° API) than conventional petroleum crude, is being economically recovered for commercial sale. During the recovery process and prior to the transport to refineries for upgrading, the heavy oil receives preliminary treatment for water and solids removal to generally achieve basic sediment and water (BS & W) content less than 0.5% by volume and chloride content less than 30 ppm (wt), more recently, the chloride content has been decreased to less than 10 ppm. Water content of the treated heavy oil typically is required to be 0.3% by volume or less.
Conventional crude oil treatment methods were proven to be ineffective with respect to heavy oil until the advent of the technology set forth in U.S. Pat. Reissue No. 33,999, Clare et al., reissued Jul. 21, 1992 and Canadian Patent 1,302,937, Clare et al., reissued on Jun. 9, 1992. These patents describe a simple apparatus which can be located in remote oil producing areas for dehydrating heavy oil with low risk of foaming and unstable operating, while continuously achieving dry oil which exceeds requisite specifications. These dehydrators were found to be restricted to feed oil water content of less than 5% water cuts and susceptible to foaming and process instability during high water feed rates. Throughout the operation of several of these dehydrators known from practicing the technology in Pat. Re No. 33,999 and U.S. Pat. No. 1,302,937, areas for improvement were discovered to overcome the limitations of feed oil water content and unstable operation caused by pretreatment upsets.
Further refinements in the crude oil processing were developed by Kresnyak and Shaw in U.S. Pat. No. 6,372,123, issued Apr. 16, 2002.
In the dehydration of crude, significant fluctuations in the temperature in the dehydrator can be experienced since heat enthalpy is continuously removed in order to vaporize the water in the crude oil. Kresnyak and Shaw recognized that this heat enthalpy needed to be restored in order to stabilize the temperature within the dehydrator and more particularly, the temperature of the heated dehydrated crude oil within the dehydrator. By recirculating at least a portion of the dehydrated crude and contacting this with the source of crude oil immediately below the vaporizing surface in the dehydrator, a substantially uniform temperature of the vaporizing surface in the dehydrator was realized. Accompanying advantages were immediately realized in terms of reduced foaming within the dehydrator and less process impediments.
Further, additional problems have been experienced in dehydration techniques in that although dehydrated heavy oil is achieved, high concentrations of suspended solids, such as clay and silica and dissolved compounds such as chlorides remain in the treated oil. These undesirable compounds continue to create many problems in pipeline transportation systems and refinery facilities to the extent that they depreciate the commercial valve of heavy oil.
It has been found in field applications that mineral salts, silica, clay inter alia that remain in the dehydrated crude promote corrosion cracking in stainless steel components and induce scale accretion and/or fouling of surfaces critical to efficient and consistent operation of the apparatus in the refiner and pipeline systems. Generally speaking, the salt crystals mix with the oil and coalescence results to form larger crystals which can pass through the refinery desalination equipment.
In view of the fact that the dehydration process is a water removal system for the crude oil, it then follows that mineral concentration is a distinct drawback. Advances have been made in respect of this limitation and in particular, dehydrators have been modified to include a demineralization/solid removal unit operation to avoid any concentration of the latter within the treatment circuit.
Having set forth the background of the dehydration technology, one of the remaining process limitations that was discovered relates to the use of diluent in the system. Unfortunately, within the processes and particularly the first generation dehydration technology, a significant amount of diluent was required. Typically, 20% to 50% by volume diluent was required in order to effect the first generation processes. Clearly, this has significant impact on the available volume within the pipeline and as a natural consequence, pipelines either had to be 50% larger in order to have the same efficiency in the absence of the diluent or, the process was inherently 20% to 50% less efficient. Although a detriment, first generation systems had inherent advantages such as good separation and operated at significantly cooler temperatures.
In flash treatment systems subsequently developed, the process produced dry oil, did not involve the extensive use of many pieces of equipment to handle different unit operations and therefore was more affordable and more importantly, did not require any diluent. Despite the significant advantages, flash treatment systems were not equipped to handle chloride problems as indicated above.
It would be advantageous if methodology could be developed which unifies all of the positive attributes of first generation processes with flash treatment process without the disadvantages and in particular, without the requirement for a diluent make up. The present invention is directed to a union of all of the positive attributes of existing systems and conveniently provides for high diluent recycle.
Accordingly, one object of the present invention is to provide advances to overcome the limitations encountered by the previous art.